[server] drop system_source_cnx for a simpler internal API system_source_cnx() can be replaced with source_cnx(). If system_source_cnx() is called with dbms_system_base=False, it is strictly equivalent to calling source_cnx() with the same source, privileges and interactivity. If system_source_cnx() is called with dbms_system_base=True, it is equivalent to calling source_cnx() with the system database and the same source, privileges and interactivity. No use of the default value for 'special_privs' was found in the code.

[server/sources/native] refactor to remove useless injection